Disclosed herein are improved photosensitive imaging members. More specifically, disclosed herein are imaging members exhibiting improved electrical and photodischarge properties and improved lateral charge migration resistance. One embodiment is directed to an imaging member comprising a conductive substrate, a photogenerating layer comprising a photogenerating material in contact with the substrate, a first charge transport layer in contact with the photogenerating layer, said first charge transport layer comprising a charge transport material and a polymer containing carboxylic acid groups or groups capable of forming carboxylic acid groups, and a second charge transport layer in contact with the first charge transport layer, said second charge transport layer comprising a charge transport material and a hydroquinone antioxidant, wherein the first charge transport layer is situated between the second charge transport layer and the photogenerating layer.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known, and is commonly referred to, variously, as electrophotography, xerography, electrophotographic imaging, electrostatographic imaging, and the like. The basic electrophotographic imaging process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, entails placing a uniform electrostatic charge on a photoconductive imaging member (also commonly referred to as a photoreceptor), which can be in the form of a plate, drum, belt, or any other desired form, exposing the imaging member to a light and shadow image to dissipate the charge on the areas of the imaging member exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material known as toner. In the Charge Area Development (CAD) scheme, the toner will normally be attracted to those areas of the imaging member which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. In the Discharge Area Development (DAD) scheme, the toner will normally be attracted to those areas of the imaging member which are uncharged, thereby forming a toner image corresponding to a negative of the electrostatic latent image. The developed image can then be transferred to a substrate such as paper. The transferred image can subsequently be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment.
Photoreceptor materials comprising inorganic or organic materials wherein the charge generating and charge transport functions are performed by discrete contiguous layers are known. Additionally, layered photoreceptor members are disclosed in the prior art, including photoreceptors having an overcoat layer of an electrically insulating polymeric material. Other layered photoresponsive devices have been disclosed, including those comprising separate photogenerating layers and charge transport layers as described in U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference. Photoresponsive materials containing a hole injecting layer overcoated with a hole transport layer, followed by an overcoating of a photogenerating layer, and a top coating of an insulating organic resin, are disclosed in U.S. Pat. No. 4,251,612, the disclosure of which is totally incorporated herein by reference. Examples of photogenerating layers disclosed in these patents include trigonal selenium and phthalocyanines, while examples of transport layers include certain aryl diamines as illustrated therein.
In addition, U.S. Pat. No. 3,041,167 discloses an overcoated imaging member containing a conductive substrate, a photoconductive layer, and an overcoating layer of an electrically insulating polymeric material. This member can be employed in electrophotographic imaging processes by initially charging the member with an electrostatic charge of a first polarity, followed by exposing it to form an electrostatic latent image that can subsequently be developed to form a visible image.
Additional conventional photoreceptors and their materials are disclosed in, for example, U.S. Pat. Nos. 5,489,496, 4,579,801, 4,518,669, 4,775,605, 5,656,407, 5,641,599, 5,344,734, 5,721,080, 5,017,449, 6,200,716, 6,180,309, and 6,207,334, the disclosures of each of which are totally incorporated herein by reference.
U.S. Pat. No. 7,267,917 (Tong et al.), the disclosure of which is totally incorporated herein by reference, discloses a charge transport layer composition for a photoreceptor including at least a binder, at least one arylamine charge transport material, e.g., N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine, and at least one polymer containing carboxylic acid groups or groups capable of forming carboxylic acid groups. The charge transport layer forms a layer of photoreceptor, which also includes an optional anti-curl layer, a substrate, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and optionally one or more overcoat or protective layers.
While known materials and devices are suitable for their intended purposes, a need remains for improved photosensitive imaging members. For example, it is desirable to increase the surface discharge speed of the photoreceptor to allow for higher speed printing applications. It is also desirable to minimize any Lateral Charge Migration (LCM) and to minimize changes in the electrical characteristics of the photoreceptor during prolonged electrical cycling. Lateral charge migration is the movement of charges on or near the surface of an almost insulating photoconductor surface, and has the effect of smoothing out the spatial variations in the surface charge density profile of the latent image. It can be caused by a number of different substances or events, such as ionic contaminants from the environment, naturally occurring charging device effluents, and the like, which cause the charges to move. LCM can occur locally or over the entire photoconductor surface. As a result, some of the fine features present in the input image may not be present in the final print. Increasing the print speed without changing the print engine architecture reduces the time from the exposure stage to the development stage, which reduces the time available for the photoreceptor's surface to discharge. If the charges are still in transit, a higher surface voltage on the photoreceptor remains during development, which consequently has a negative impact on print quality. To solve this problem, high discharge rate charge transport molecules have been tested in the hopes of enabling increased print speeds. N,N,N′N′-Tetra(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine is one example of a high discharge rate charge transport molecule. High discharge rate charge transport molecules, however, also tend to exhibit undesirably high lateral charge migration, and attempts at reducing the LCM tend to entail some decrease of discharge rate to improve LCM. It would be highly desirable to reduce LCM while either leaving discharge rate unchanged or improving discharge rate.
As used herein, “discharge rate” refers to the voltage drop over time and is based upon a discharge over a discharge interval at a given light intensity, wherein discharge is defined as the voltage drop or difference between the initial surface voltage before light exposure and the surface voltage after light exposure at the end of the discharge interval. Discharge interval is defined as the time period from the light exposure stage to the development stage (which is essentially the time available for the photoreceptor surface to discharge from an initial voltage to a development voltage) and light intensity is defined as the intensity of light used to generate discharge in the photoreceptor. The exposure light intensity influences the amount of discharge, and increasing or decreasing light intensity will respectively increase or decrease the voltage drop over a given discharge interval.